Energy-saving has currently been required in various fields. Also in an internal combustion engine of an automobile and the like, energy-saving, i.e., low fuel consumption has been strongly desired.
A lubricating oil, which is used for lubrication of sliding parts in the internal combustion engine, generally exhibits decreasing viscosity in accordance with increasing temperature. However, viscosity retention of the lubricating oil at a high temperature is also significant in order to maintain lubricity and wear resistance of bearings. For instance, an engine oil of SAE (Society of Automotive Engineers) viscosity grade 30 is required to maintain 2.9 mPa·s or more of a high-shear viscosity at 150 degrees C.
Moreover, a viscosity around 80 degrees C. is also reported to affect fuel consumption, where low fuel consumption is more achievable as the viscosity around 80 degrees C. decreases. Accordingly, the lubricating oil having a high viscosity index is favorable. In most lubricating oils, various additives are mixed with a base oil. A polymer compound called a viscosity index improver is often added thereto in order to increase the viscosity index (see, for instance, Patent Literature 1).